The invention relates to a post amplifier that receives the differential output of a product detector and provides a single-ended output that is referenced to ground. That is the output swings above and below ground in response to the modulation on the modulated carrier signal, but for quiescent conditions is at ground potential. Product detectors are well-known in the form of synchronous detectors for responding to amplitude modulated signals and as phase or frequency detectors well-known in the FM receiver art
My U.S. Pat. No. 3,908,171 issued Sept. 23, 1975, and is titled "AMPLIFIER CIRCUIT SUITABLE FOR AMPLIFYING DIFFERENTIAL INPUT SIGNALS AN PROVIDING A SINGLE ENDED OUTPUT SIGNAL". The title of this patent describes the circuit. My U.S. Pat. No. 4,275,419 issued June 23, 1981, and is titled "VIDEO AMPLIFIER WITH NOISE INVERSION". This patent relates to a synchronous video detector with a noise-cancelling post-detection amplifier. The teaching in these two patents is incorporated herein by reference.